grammatical-numberorthographypronunciation-vs-spelling
The plural form of leaf is leaves, although according to Merriam-Webster leafs is also correct. Dwarf can be pluralized as either dwarfs or dwarves. Conversely, the words roof and safe are pluralized as roofs and safes.
Is there any logic underpinning these variations? Is there a historical trend where words that were previously pluralized as -ves are now being increasingly pluralized as -fs?
I take the real question here to be: "Why do some people pedantically cling to dying forms?" That's a good question. I think the answer is relatively straightforward.
People who want to present an air of education and in general lay claim to upper class privilege are the ones who tend to do this. It's largely because it is an index of education and a high degree of literacy (either that or role playing games, which is somewhat different, but still primarily an upper middle class past time...).
There is a lot more to it than that, of course. There is an intricate set of language ideologies which give rise to this kind of behavior. But the short answer is that in using such forms people attempt to identify with culturally and economically powerful (hence linguistically conservative) groups .
Everyone does this to some degree, of course.
I did some research using the Corpus of Historical American English to see if I could track the history of these words. Each of these words has a different story to tell.
See the raw data (Google Docs)
In the early 1800s, dreamt was more common than dreamed but by the mid-1800s, dreamed was much more common and has stayed so since. While there is nothing wrong with continuing to use dreamt, dreamed is definitely the more common form.
See the raw data (Google Docs)
Leaped has long been more popular than leapt, though leaped has been in decline since 1900, and leapt has been on the increase since 1950, and today they are about equally common. It is likely that if the current trends continue, leapt will become decisively more common than leaped within a decade or two. Indeed, in the Corpus of Contemporary American English, leapt has 484 incidences for 2005-2010 and only 460 for leaped. So, both are about equally common these days and you are in good company if you prefer leapt.
Neither COCA nor COHA have any results for sweeped, nor does any dictionary I checked list sweeped as a possible past tense form for sweep. Sweeped doesn’t appear to have had any currency in American English since 1810. Google reports only 43,000 results for sweeped compared with 78,000,000 for swept.
See the raw data (Google Docs)
Apparently, lighted was much more popular than lit, from the early 1800s until about 1940. During this time lit was steadily gaining popularity, while lighted began a precipitous decline in 1940. Today, lit is much more popular than lighted, so if you prefer lit, you are in very good company. However, there would be trouble objecting to lighted on historical grounds, as lighted was by far the most common form until the 1940s.
Best Answer
So, the f->v shift can be traced back to Old English, where v wasn't its own letter, but merely an allophone of f. The /v/ pronunciation was used when it was placed between vowels or voiced consonants, and the /f/ pronunciation was used otherwise.
So the declination from wīf to wīfes meant that the actual pronunciation of the f went from /f/ to /v/ (much like today today), because it became positioned between two voiced vowels (I should point out, in OE wīfes was a two-syllable word [wiːvɛs]).
Now, over time, a few things happened:
So we have a couple competing forces:
With the v sound now entrenched in the spelling of many words, the idea of the f->ves for pluralization became a "rule" and was carried on and used by analogy when forming related/similar words.
However, with the lack of a vowel sound in the final syllable, there was no longer a need to force the voicing of the labial fricative, leaving us with two viable pronunciations/spellings: the unvoiced pair [fs] (-fs) or the voiced pair [vz] (-ves)
Now, I can't find a good source for determining which word used which one, so treat the following as my own supposition:
My expectation then is that when a particular word entered the lexicon would greatly influence which option is chosen. Words entering in Old and Middle English (and words derived from or related to them) would be much more likely to use the -ves option, while words entering later, particularly if they enter as loanwords from a Romance language, etc. would prefer the more regular -fs.
Additionally, now that there's no phonetic requirement to voice the consonant, we're seeing linguistic regularization kicking in, slowly pushing the -ves pluralization out in favor of the more regular -fs option. So forms like rooves / hooves start to give way to roofs / hoofs and depending on where they are in that process, you see that one form may be preferred over another, or they may both be equally viable.